There were some twitter threads by devs I was following with a fair amount of analysis on why Epic chose this percentage. People concluded that it's actually very close to the breakeven point and that Steam's margins are nowhere near what people think they are. Once you factor in third party keys, overhead and etc, Steam can't really operate at this margin in the long term. Neither can Epic by the way, it's ability to do so now is due to its lack of equivalent features resulting in lower overhead and its Fortnite warchest. Epic did a lot of market research and sales analysis before they started this, the whole SteamSpy thing was especially telling.
Anyways I think publishers will largely take Epic's free money and then continue to move more of their IP into their own little silos (uPlay, Origin, etc) anyway, I'm not sure this is going to keep panning out much longer. This sounds like Sweeney playing the PR game and trying to come across the good guy fighting for the little consumer or developer. I'm not really convinced based on what I've seen so far but ultimately I don't really care where I buy something, I just want competitive features and pricing.